1. Field
The present invention relates generally to communications, and more specifically to a novel and improved cell acquisition process that reduces the impact of frequency error.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other multiple access techniques. A CDMA system provides certain advantages over other types of systems, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the Wideband Code Division Multiple Access (W-CDMA) standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “C.S0002-A Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “C.S0024 cdma2000 High Rate Packet Data Air Interface Specification” (the cdma2000 standard), and (4) some other standards.
Pseudorandom noise (PN) sequences are commonly used in CDMA systems for spreading transmitted data, including transmitted pilot signals. The time required to transmit a single value of the PN sequence is known as a chip, and the rate at which the chips vary is known as the chip rate. CDMA receivers commonly employ RAKE receivers. A rake receiver is typically made up of one or more searchers for locating direct and multipath pilots from one or more base stations, and one or more multipath demodulators (fingers) for receiving and combining information signals from those base stations. A base station and its coverage are often collectively referred to as a “cell.”
Inherent in the design of direct sequence CDMA systems is the requirement that a receiver must align its PN sequences to those of a base station. For example, in IS-95, each base station and subscriber unit uses the exact same PN sequences. A base station distinguishes itself from other base stations by inserting a unique time offset in the generation of its PN sequences (all base stations are offset by an integer multiple of 64 chips). A subscriber unit communicates with a base station by assigning at least one finger to that base station. An assigned finger must insert the appropriate offset into its PN sequence in order to communicate with that base station. An IS-95 receiver uses one or more searchers to locate the offsets of pilot signals, and hence to use those offsets in assigning fingers for receiving. Since IS-95 systems use a single set of in-phase (I) and quadrature (Q) PN sequences, one method of pilot location is to simply search the entire PN space by correlating an internally generated PN sequence with different offset hypotheses until one or more pilot signals are located.
Other systems, such as W-CDMA systems, differentiate base stations using a unique PN code for each, known as a primary scrambling code. The W-CDMA standard defines two Gold code sequences for scrambling the downlink, one for the in-phase component (I) and another for the quadrature (Q). The I and Q PN sequences together are broadcast throughout the cell without data modulation. This broadcast is referred to as the common pilot channel (CPICH). The PN sequences generated are truncated to a length of 38,400 chips. The period of 38,400 chips is referred to as a radio frame. Each radio frame is divided into 15 equal sections referred to as slots.
The first step taken by a radio communication device such as a mobile radio unit when commencing communications in a CDMA system is to identify the transmitting base stations or cells in terms of their primary scrambling code and the corresponding frame timing. Prior to commencing communications with a base station, the mobile unit has to synchronize itself with the timing reference of a base station. This process is commonly referred to as cell search. Once the primary scrambling code and frame timing of the target cell has been identified, the mobile unit sets up signaling and user (voice or data) channels to communicate with the base station.
It is possible to search for W-CDMA base stations in the manner described for IS-95 systems, described above. That is, the entire PN space can be searched offset by offset (38,400 of them) for each of the 512 primary codes. However, this is not practical due to the excessive amount of time such a search would require. Instead, the W-CDMA standard calls for base stations to transmit two additional synchronization channels, the primary and secondary synchronization channels, to assist the subscriber unit in searching efficiently.
As a result, W-CDMA search can be performed in three main steps, as follows:
Step 1. Slot synchronization: During this first stage of the cell search procedure, the mobile unit uses the Primary Synchronization Channel's (P-SCH) Primary Synchronization Code (PSC) to acquire slot (e.g., slots occur in 666 μs time intervals) synchronization to a cell as shown in step 102 of FIG. 1. The PSC is a 256-chip length code transmitted during the first 256 chips of each 2,560-chip slot, and all cells transmit the same PSC. The PSC is useful for detecting the presence of a base station or cell, and once it is acquired, slot timing is also acquired. The process involves correlating time shifted versions of the PSC with the incoming signal and detecting peaks. FIG. 2 shows one possible framing structure for the downlink in W-CDMA. This framing structure is used for a downlink dedicated physical channel (DPCH), which carries user-specific data for a terminal. The timeline for data transmission is divided into radio frames. Each radio frame is identified by a 12-bit system frame number (SFN) that is transmitted on a control channel. The SFN is reset to zero at a specific time, is incremented by one for each radio frame thereafter, and wraps around to zero after reaching the maximum value of 4095.
Each radio frame has a duration of 10 milliseconds (ms) and is further partitioned into 15 slots, which are labeled as slot 0 through slot 14. Each slot includes two data fields (Data1 and Data2) used to send user-specific data, a transmit power control (TPC) field used to send power control information, a transport format combination indicator (TFCI) field used to send format information (e.g., bit rate, channelization code, and so on), and a pilot field used to send a pilot.
Step 2. Frame synchronization and code-group identification: During the second step of the cell search procedure as shown in step 104 of FIG. 1, the mobile unit uses the secondary synchronization code (SSC) on the secondary synchronization channel (S-SCH) to find frame synchronization and identify the code group of the cell found in the first step. All primary scrambling codes used in W-CDMA systems are divided into groups of eight, and the code group is encoded into a sequence of SSCs. Correlating the received signal with all possible SSC sequences, and identifying the maximum correlation value accomplishes this. Since the cyclic shifts of the sequences are unique, the code group as well as the frame synchronization is determined.
Step 3. Scrambling code identification: During the third and final step of the cell search procedure as shown in step 106 of FIG. 1, the mobile unit determines the exact primary scrambling code used by the cell it has found. The primary scrambling code is typically identified through correlation over the common pilot channel (CPICH) with all eight codes within the code group identified in the second step, in a window around the frame timing provided in Step 2.
Steps 2 and 3 are performed until the correct scrambling code is identified, upon which the cell is acquired.
Frequency error affects acquisition performance in a CDMA system. Like the PSC, the SSC is available the first 256 chips of each 2,560-chip slot. Since at least 15 slots (i.e., 1 frame) and usually about 45 slots (i.e., 3 frames) are required to identify a code group in Step 2, a large number of samples must be taken. Every time a sample is taken, frequency error will lead to phase error and sampling time error, which accumulates over the entire search duration. Thus, over time, a given frequency error will cause both the phase error and the error in sampling time to accumulate. The error accumulation may, for example, cause the peak position to drift, thus impacting search performance. Reducing the number of samples taken will lead to reducing the accumulated phase and timing errors.
The benefits of reduced phase and sampling time errors are clear, and some issues associated with searching in asynchronous systems, such as W-CDMA, have just been highlighted, including initial cell acquisition. There is therefore a need in the art for improved search techniques for asynchronous systems, including initial cell acquisition.